1. Field of the Invention
The present invention relates to a parison molding apparatus, and more particularly to a method and an apparatus for molding a parison by using a mixture containing a main resin material and auxiliary resin material consisting of at least two kinds of additional materials, such as polyamide and adhesive resin. More specifically, the apparatus and method of the present invention are improved in that the structure of layer and dispersion of the auxiliary resin are uniform in a circumferential direction of the parison and at a weld portion of the parison.
2. Prior art
There are many methods and apparatuses for molding a parison by using a mixture of different kinds of materials. Of those methods and apparatuses, the apparatus as shown in FIGS. 4 through 6 is most widely used.
In FIG. 4, reference numeral 1 designates a parison head of the accumulator type coupled with a nozzle 3 of an extruder 2. A tubular manifold 4, which is formed in the parison head 1, communicates with the nozzle 3.
The manifold 4 includes an accumulator chamber 7 defined by a piston 6, which is vertically moved by a cylinder 100. A tapered core 9 is located under the accumulator chamber 7. The tapered core 9 is connected to a rod 8, which is vertically movable along the axis of the parison head 1. The thickness of a parison 11, which is discharged through a discharge port 10, is controlled by the vertical movement of the tapered core 9. The discharge port 10 communicates with the accumulator chamber 7.
In the parison molding apparatus thus constructed, molten resin extruded from the extruder 2 is supplied to the manifold 4 through the nozzle 3. In the manifold 4, the molten resin bifurcates, and the bifurcated molten resin flows along the wall 4a of the manifold 4 in the directions of arrows in FIG. 5, and joins each other at a portion located in opposition to the nozzle 3, thereby forming a weld part 5. Then, the piston 6 is pushed upward, and the molten resin is accumulated in the chamber 7.
When the piston 6 is pushed downward through the cylinder 100, the molten resin stayed in the accumulator chamber 7 is extruded out of the chamber through a discharge port 10, in the form of the parison 11.
Another type of the parison molding apparatus, not shown, has been used by some manufacturers. In the apparatus, the parison head 1 is of the double flow type, which has two layers, the inner layer and the outer layer, causing each weld part in different phases.
The conventional parison molding apparatus and its methods have the following disadvantages because of its construction.
In the conventional structure, since the manifold 4 consists of a ring-like closed path, the molten resin extruded through the nozzle 3 from the extruder bifurcates, and the bifurcated molten resin join each other in the weld part and are bonded together in the radial direction of the parison head 1 (FIG. 5). When the container thus flow molded is filled with liquid, e.g., gasoline, there is a possibility that the gasoline permeates through the weld part 5 of the container.
Attempt has been made to prevent the gasoline permeation. In the attempt, the main resin material is mixed with an additional material for preventing such a permeation. An auxiliary material is obtained, for example, by mixing polyamide and the adhesive resin. The main resin material and, polyamide and adhesive resin, which are mixed in the ratio of 7:3. The molten resin bifurcates to flow in two directions along the wall of the manifold, as shown in FIG. 6. As the molten resin goes away from the nozzle, the extruding pressure acting on the molten resin becomes weaker. Accordingly, the large amounts of auxiliary resin material is likely to stay in a portion closer to the nozzle. In other words, less amounts of the auxiliary resin material reach the weld portion.
Since the extruding pressure is low in the weld portion, the amounts of the extruded additional materials are small. The amounts of the additional materials, which are dispersed in the form of film and are used for preventing the liquid penetration, are too small to form the layer. In a container, for example, a gasoline tank, which is molded by blow molding under this condition, the weld part thereof exhibits poor barrier property. Accordingly, the gasoline easily to penetrate into the weld part, causing gasoline leakage.
The prior art parison head of the double layer type requires a complicated structure. In this sense, it is unsuitable for the mass production manufacturing.
For example, two methods for molding plastic gasoline (fuel) tanks for cars have been known. In the first method, a) main resin material for keeping the shape of the tank and for securing the required strength (rigidity), b) auxiliary resin material for increasing the barrier property or improving the gasoline permeability, and c) adhesive for bonding the materials a) and b) above together are molded by a multi-layer blowing mold method (e.g. Unexamined Japanese Patent Publication Hei. 2-286219). In the second method, the materials a), b), and c) are previously mixed and molded by a blowing mold method. To carry out the first method, a plural number of extruders are required. This results in a complicated machine structure and operation, and expense of the machine.